1. Field of the Invention
The present invention relates to the field of image display systems and more particularly to techniques for displaying video and still pictures on a monitor where the aspect ratio of the monitor differs from the aspect ratio of the pictures.
2. Related Art
Motion video and still picture information is often captured (e.g., recorded on film) with one aspect ratio and then must be displayed on a system with a different aspect ratio. An example of when this occurs is when movies are converted to video signals to be displayed on television monitors. Each frame in a "modern" movie has a display aspect ratio of 9:16; that is, the ratio of the height to the width of the display area on the screen on which the movie is to be viewed is 9 to 16. The ratio of the height to the width of a television display area (in the U.S.) is about 3:4. Therefore, some processing must be performed to display a movie on a television set.
FIG. 1 illustrates the more common method of showing movies on television; we refer to this method as the "pan-scan" method. For the pan-scan method, a subpicture from each picture is selected, and the rest of the picture is discarded. Specifically, areas on the leftmost and rightmost sides of the pictures are discarded, and the picture that remains has an aspect ratio of 3:4, so it may be viewed on a television set. A video signal containing only the remaining rectangle is generated, and this signal is broadcasted or stored on a video tape. The location of the "display rectangle" within the entire picture may vary from picture to picture. This method is referred to as the pan-scan method because the retained rectangle is often moved from left to right or vice-versa as the camera pans so that the more important parts of the scene are retained.
FIG. 2 illustrates the "letterbox" method of displaying movies on television. For this method, a blank area is appended to the top and bottom of each picture, so that the aspect ratio of a picture made up of the original picture plus the blank areas is 3:4. These pictures are then used to generate a video signal that is then broadcasted or stored on a video tape.
When a movie is broadcast (through the air or cable) or stored on a video tape, it is typically converted to the correct aspect ratio using either the pan-scan or letterbox method and then broadcasted or stored on video tape. In this case, the television receiver or tape player does not have to do any aspect ratio conversion; the pictures already have the correct aspect ratio.
Technological advances in digital transmission networks, digital storage media, Very Large Scale Integration devices, and digital processing of video and audio signals are converging to make the transmission and storage of digital video economical in a wide variety of applications. Because the storage and transmission of digital video signals is central to many applications, and because an uncompressed representation of a video signal requires a large amount of storage, the use of digital video compression techniques is vital to this advancing art. In this regard, several international standards for the compression of digital video signals have emerged over the past decade, with more currently under development. These standards apply to algorithms for the transmission and storage of compressed digital video in a variety of applications, including: video-telephony and teleconferencing; high quality digital television transmission on coaxial and fiber-optic networks as well as broadcast terrestrially and over direct broadcast satellites; and in interactive multimedia products on CD-ROM, Digital Audio Tape, and Winchester disk drives.
Among these standards, the MPEG-2 standard provides a tool for creating a digital representation of a video sequence that can be displayed on monitors with different aspect ratios. Specifically, an MPEG-2 sequence can contain information to reconstruct the entire area of each picture in a video sequence and also contain information about what should be displayed if the monitor has an aspect ratio that is different from the aspect ratio of the coded pictures. For example, a movie can be compressed so that each frame (with a 9:16 aspect ratio) can be completely reconstructed, but the location of the display rectangle is also provided for each frame. In this case, an MPEG-2 decoder connected to a display device with a 9:16 aspect ratio would display each picture in its entirety, but an MPEG-2 decoder connected to a display with a 3:4 aspect ratio would extract and display only the display rectangle. Thus, the MPEG-2 standard provides a tool for encoding a video sequence with a given aspect ratio and displaying that sequence on a device with a different aspect ratio in pan-scan form by performing pan-scan conversion immediately before display.
Another conventional system of handling aspect ratio conversion involves proving a widescreen (e.g. 9:16) television receiver with multiple display modes. If the television receives a regular 3:4 aspect ratio signal, the picture is cropped vertically and displayed on the entire screen; in this case, the entire screen is used but not all of the active picture is displayed. If the television receives a signal with a 3:4 aspect ratio but with an active area that is 9:16--e.g., a movie that has been converted to letterbox format for display on a television set with a 3:4 aspect ratio and is broadcast with blank areas on the bottom and top of each picture--the television again crops the picture vertically and displays the resulting image on the entire screen; in this case, the entire screen is used and the active part of the scene is retained. Finally, if the television receives a signal from a source having a display aspect ratio of 9:16 the entire signal is displayed without cropping. It is noted that for all of the display modes in the above described system, the entire screen area is used to display all or part of the active part of each picture.